The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to the collection of marine seismic data in deep water environments.
Marine seismic exploration requires acoustic source generators for delivering energy to subsurface geologic formations and boundaries. The acoustic energy is discharged along shot lines in the desired survey region and is reflected by the subsurface formations and boundaries. The reflected energy propagates upwardly and is detected with hydrophones or bottom cable geophones. Data from selected shot patterns are processed to construct an overall acoustic image of the subsurface geologic formations.
Bottom cable systems use geophones or hydrophones laid on the sea floor. Cable crews assemble each cable from multiple cable sections. Bottom cables are deployed and retrieved with the assistance of powerful linear traction engines and other machines. In shallow water up to one hundred meters deep, one or more vessels deploy the geophones and cable in the selected locations and retrieve the geophones and cables after the selected area is surveyed.
Bottom cables are used in relatively shallow water depths to communicate between underwater seismic equipment located on board a marine seismic vessel. Bottom cables are heavy because the sensor packs are heavy and depend upon gravity to achieve physical coupling between the geophones and ocean bottom. Additionally, such cables contain insulated wires for transmitting electrical power and signals between submarine seismic assets and the seismic vessel, and a heavy strength member is required to lift the submarine assets from the ocean bottom and to resist hydrodynamic and mechanical loads imposed on the cable during such recovery. The weight, size and reliability issues regarding bottom cables, together with the operational expense of deploying and retrieving bottom cables, effectively prevents conventional bottom cable systems from being economically deployed in deep water.
Conventional bottom cable systems are not useful in deep water because such systems require expensive, powerful and complex handling equipment and such systems have a large cross-section which produces significant hydrodynamic drag and correspondingly long deployment and retrieval times. Conventional bottom cable systems require significant repair costs and operational downtime due to frequent mechanical and electrical failure of cable components resulting from repeated exposure to stress loading experienced during cable retrieval from the ocean floor.
In addition to technical difficulties inherent in bottom cable systems, the size, weight and handling requirements of bottom cable systems are expensive and difficult to implement in deep water environments. To overcome these limitations, marine seismic streamer vessels are conventionally used in deep water to perform seismic surveys. Marine seismic vessels tow acoustic energy sources such as compressed air guns through the water. The vessels also tow one or more seismic streamer cables along the selected survey line. The streamers typically range between three and eight kilometers long, with tail buoys attached to the free streamer ends. Each streamer contains multiple hydrophones which receive the reflected energy emitted by the energy source, and the hydrophones are wired together in receiver groups regularly spaced along the streamer. To account for vessel movement, data recording and processing calculations require time and position correlations for each active component of the seismic data gathering system.
Variations in lateral spacing between adjacent streamers due to environmental forces and vessel course changes can adversely affect the collected data. The actual spacing between receiver groups is critical for an accurate analysis of geophysical data. Because the acoustic energy reflections propagate through the water in a towed streamer system, noise significantly distorts the reflected energy. The problems associated with undesirable noise is well known. For example, U.S. Pat. No. 4,970,696 to Crews et al. (1990) disclosed a three dimensional seismic survey system having multiple seismic receivers. Undesirable noise was characterized with uniform sampling intervals and the recorded responses were processed to remove the undesirable noise. In other systems, additional processing is required to account for additional noise and the quality of maps created from the processed data is reduced.
Various systems have been proposed to collect data in a marine environment and to return the data to the water surface. U.S. Pat. No. 4,007,436 to McMahon (1977) disclosed a flexible sheet for holding hydrophones. U.S. Pat. No. 4,692,906 to Neeley disclosed an ocean bottom seismometer having a weighted skirt which also provided a buoyant space for retrieving the seismometer to the water surface. U.S. Pat. No. 5,189,642 to Donoho et al. (1993) disclosed a seafloor seismic recorder having a chassis which lowered geophones into contact with the seafloor. A geophone package was embedded into the seafloor and a control package and chassis was raised above the seafloor surface with a leg extension to isolate such components from the geophone package. A ballast ring returned the geophone package to the water surface. Additionally, U.S. Pat. No. 5,696,738 to Lazauski (1997) disclosed a sensing device in contact with the seafloor.
Other systems have been developed to operate and to collect seismic data from multiple recorders. U.S. Pat. No. 4,281,403 to Siems (1981) disclosed a plurality of remote seismic recording units activated with local clocks in each local recording unit, together with a master clock in a central station. U.S. Pat. No. 5,623,455 to Norris (1997) disclosed remote units connected to a plurality of receivers for collecting and transmitting seismic data over a selected transmission channel. U.S. Pat. No. 5,724,241 to Wood et al. (1998) disclosed a distributed data acquisition system having a plurality of recorders for detecting and recording seismic data. The data was collected and continuously collected by data acquisition modules.
A need exists for an improved deep water seismic data collection system. The system should be economic to deploy and should preserve the quality of collected data.
The present invention provides a system for collecting seismic data from geologic formations underlying water. The system comprises a plurality of housings deployable in the water, wherein each housing has a first end having a hydrodynamic profile for facilitating descent of the housing through the water and into contact with the geologic formations. A controller is engaged with each housing for reconfiguring the housing after the housing contacts the geologic formations, and one or more marine seismic sensors are engaged with each housing for detecting seismic data and for identifying the orientation of the sensor. An actuator facilitates retrieval of each marine seismic sensor to the water surface.
The method of the invention comprises the steps of operating a vessel in water, of deploying a plurality of housings and engaged marine seismic sensors into the water until each housing contacts the geologic formations, of operating a controller engaged with each housing to reconfigure the housing after the housing contacts the geologic formations, of operating the marine seismic sensors to detect seismic source energy reflections and to record seismic data representing such reflections, and of operating an actuator to facilitate retrieval of each marine seismic sensor to the water surface.